The present invention relates to a method and an apparatus for manufacturing a rolling die when manufacturing an hourglass worm.
A reduction gear mechanism using an hourglass worm is known in the art. The hourglass worm has more teeth that mate with a cylindrical worm than a worm wheel, and is thus used for vehicles, for example, in small-size motors, which incorporate reduction gear mechanisms, and high-load reduction gears.
Generally, an hourglass worm is manufactured through a cutting process performed by a screw cutting machine or a gear hobbing machine and the like, or a grinding process using a grindstone. However, in such processing methods, the processing time is long, and is difficult to produce the hourglass worm in large quantities. A rolling process using a die is thus performed to produce the hourglass worm in large quantities. Japanese Laid-Open Patent Publication No. 2003-320434 describes a method for manufacturing a die used in rolling.
The above publication describes a method for manufacturing a circular die. The circular die is manufactured by forming teeth grooves to roll an hourglass worm on the peripheral surface of a disk-shaped workpiece using a rotary tool (rotary grindstone). The workpiece is formed so that when viewing a cross-section of the workpiece that includes the workpiece axis, the peripheral surface is arcuate and projects radially outward with a radius of curvature equal to that of the round surface at the teeth roots of an hourglass worm.
The rotary tool is attached to an oscillating unit supported by two oscillating arms. More specifically, two holders projecting towards the workpiece are arranged in the oscillating unit. The rotary tool is rotatably supported by the holders. A drive motor for rotating the rotary tool is mounted on one of the holders. An auxiliary motor for inclining the rotary tool is mounted on the oscillating unit at the surface opposite the workpiece so that the axis of the rotary tool inclines with respect to the axis of the workpiece by the lead angle of the hourglass worm.
When forming the teeth grooves for rolling an hourglass worm on the peripheral surface of a workpiece, the rotary tool and the workpiece are rotated with the axis of the rotary tool inclined relative to the axis of the workpiece by an amount equivalent to the lead angle of the hourglass worm. Simultaneously, the oscillating arms oscillate the oscillating unit about the center of curvature of the arc on the peripheral surface of the workpiece in synchronization with the rotation of the workpiece. This oscillates the rotary tool about the center of curvature of the arc. Further, the rotary tool is fed towards the workpiece by extending or retracting the oscillating arms or the holders. Therefore, the teeth grooves for rolling an hourglass worm is formed on the peripheral surface of the workpiece by oscillating the rotary tool in a reciprocating manner within the thickness range of the workpiece, while rotating the workpiece in forward and reverse directions in a state in which the inclination of the rotary tool is kept constant.
However, in the circular die manufacturing method described in the above publication, the rotary tool is rotated and oscillated by the oscillating unit in addition to being fed towards the workpiece to form the teeth grooves on the die. Thus, error caused by rotation of the rotary tool, error caused by oscillation of the tool, and processing error caused by feeding the tool are cumulative. This may increase the pitch error of the teeth grooves formed on the peripheral surface of the workpiece. The hourglass worm of a small-size motor requires high accuracy. Thus, it is desirable that the hourglass worm be manufactured with a circular die that is highly accurate.